Remelt lead

Besides this primary lead (manufactured from ore), secondary lead (remelted lead scrap) is of increased importance, based as it is mainly on the recychng of batteries from cars. At the beginning of the 1990s the total annual lead production in the world was 5.5 million tonnes, of which 2 milHon tonnes (thus 36%) were secondary. In 2001 the corresponding figures were 6.5 million tonnes (total), 2.8 million tonnes (secondary) and 43%. In some countries the share of the total lead production attributed to secondary lead is very high, as is shown in Table 43.4. 

The term unalloyed lead is used to describe lead with no intentional additions of other metals or alloying elements. This will include both high purity metal, with only smaU amotmts of incidental impurities, and less pure metal (such as remelt lead ). Lead alloys are produced by the addition of other elements to lead in specific amounts (from less than 0.1 per cent up to 90 per cent or more) and under controlled conditions. Sometimes those alloys with a total addition of other elements of less than 1 per cent are known separately as dilute lead alloys . 

Crude lead contains traces of a number of metals. The desilvering of lead is considered later under silver (Chapter 14). Other metallic impurities are removed by remelting under controlled conditions when arsenic and antimony form a scum of lead(II) arsenate and antimonate on the surface while copper forms an infusible alloy which also takes up any sulphur, and also appears on the surface. The removal of bismuth, a valuable by-product, from lead is accomplished by making the crude lead the anode in an electrolytic bath consisting of a solution of lead in fluorosilicic acid. Gelatin is added so that a smooth coherent deposit of lead is obtained on the pure lead cathode when the current is passed. The impurities here (i.e. all other metals) form a sludge in the electrolytic bath and are not deposited on the cathode. 

Lead—Calcium—Aluminum Alloys. Lead—calcium alloys can be protected against loss of calcium by addition of aluminum. Aluminum provides a protective oxide skin on molten lead—calcium alloys. Even when scrap is remelted, calcium content is maintained by the presence of 0.02 wt % aluminum. Alloys without aluminum rapidly lose calcium, whereas those that contain 0.03 wt % aluminum exhibit negligible calcium losses, as shown in Figure 8 (10). Even with less than optimum aluminum levels, the rate of oxidation is lower than that of aluminum-free alloys. 

AHoy scrap containing tin is handled by secondary smelters as part of their production of primary metals and alloys lead refineries accept solder, tin drosses, babbitt, and type metal. This type of scrap is remelted, impurities such as iron, copper, antimony, and zinc are removed, and the scrap is returned to the market as binary or ternary alloy. The dross obtained by cleaning up the scrap metal is returned to the primary refining process. 

The purity ot the scrap mainly determines the fraction of energy needed to produce metal from it, and the value of recycling. Clean copper scrap need only be remelted and cast to form recycled copper if the copper is contaminated with organic materials and other metals, more complex separation processes are needed that are similar to production from ores. It is easier to remelt the steel of a car driven in Arizona compared to one rusted by the road salt in snowy areas. Scrap that is produced as a by-product of metal processing can be easily recycled, and it can be collected from relatively few locations. There has been a strong effort to educate both householders and industrial users to separate scrap and return it to waste collectors, leading to a supply of reasonably separated scrap. 

Many of the following powdered metals reacted violently or explosively with fused ammonium nitrate below 200°C aluminium, antimony, bismuth, cadmium, chromium, cobalt, copper, iron, lead, magnesium, manganese, nickel, tin, zinc also brass and stainless steel. Mixtures with aluminium powder are used as the commercial explosive Ammonal. Sodium reacts to form the yellow explosive compound sodium hyponitrite, and presence of potassium sensitises the nitrate to shock [1], Shock-sensitivity of mixtures of ammonium nitrate and powdered metals decreases in the order titanium, tin, aluminium, magnesium, zinc, lead, iron, antimony, copper [2], Contact between molten aluminium and the salt is violently explosive, apparently there is a considerable risk of this happening in scrap remelting [3],... 

The SSP process is accompanied by a continuous loss of side products, such as acetaldehyde (AA) and oligomers. Both components can reduce the quality of the final products. Figure 5.25 displays the level of AA and the IV as a function of the reaction temperature. The concentration of AA decreases with increasing reaction temperature to a level of less than 3 ppm at 230 °C. Remelting, however, then leads to an increase in the AA content (Table 5.3). 

Remelt and heat it. If pure, it keeps its color after heating, and remains like a coin. If it becomes whiter, it contains silver, if it becomes rough and hard, it contains copper and tin, if it softens and blackens it contains lead. 

Twenty-two coins from the years 217 to 31 B.C. of the Roman Republic were analyzed by X-ray fluorescence for Fe, Co, Ni, Cu, Zn, As, Ag, Sn, Sb, and Pb. No evidence was found for widespread remelting of coins. The early coins are remarkable for their relatively high Co contents. Several coins have exceptionally high Pb, As, or Sb contents. Generally, the compositions of these Roman Republican coins are very different from those of Roman Imperial coins. Although few coins were analyzed, their compositions correlate reasonably well with time. Further analyses are required to determine whether composition varies with denomination and whether coins may be dated to within a few years by their chemical compositions. Microstructures of two Roman Republican coins containing lead are presented. 

The resultant cathodes must undergo a further refining step as small levels of tin and antimony can often be deposited along with the lead. These elements are removed by oxygen softening and/or caustic dressing from the remelted cathodes. To limit the level of cathode contamination, some lead refineries apply partial softening to the lead bullion before the anodes are cast. 

Known solid solution forming systems include naphthalene-theonaphthalene thiophene-benzene hexadecane-octadecane and m-chloronitrobenzene-w-fluoronitrobenzene. Further advantages of melt crystallization are the smaller volume of the liquid phase compared to the vapor phase of a substance. A smaller volume leads towards less space or less construction work, which means less capital costs. These advantages are sometimes lost if the process of crystallization and remelting is very slow therefore, the retention time in the apparatus is high. 

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